This invention concerns methods for improving the wettability and/or the adhesiveness of plastic surfaces, in particular for purposes of improving their ability to be printed, painted or glued, as well as the use of these methods.
Moreover, this invention concerns the use of metal soap-free lubricants in plastic materials to improve wettability and/or adhesiveness of their surfaces, in particular for purposes of improving their ability to be printed, painted or glued.
In general the so-called lubricants, which directly affect the rheology of the plastic melt during processing, are added to plastics, especially thermoplastics, so as to make processing as easy as possible. These lubricants serve both for improved slip of the polymer chains of the plastic over each other and, because of their surface activity, for better dispersion of additives (adjuvants). In addition, lubricants reduce the heat of friction in an extruder and aid in forming an item from an injection molding tool.
Within the scope of this invention in particular additives or adjuvants for filled plastics, especially pressed plastics or injection molding plastics, which make the fillers slip more easily and thus the pressed plastics more easily formable, are called lubricants. The lubricants themselves are insoluble in the plastics to which they are added. For further details concerning the term lubricant one can refer in particular to Römpp's Chemical Lexicon, 10th edition, Georg Thieme Publishers, Stuttgart/New York, Volume 2, 1997, p. 1553, key word: “Lubricant,” and the literature cited there, where the content in this regard is hereby incorporated in its full extent by reference.
Hydrophobic metal soaps, especially metal soaps of stearic acid (for example calcium stearate), are often used as so-called internal lubricants in the case of plastics, especially technical polyamides, because of their excellent property profile.
The term “metal soaps,” as used within the scope of this invention, means in particular the salts of the metals aluminum, barium, calcium, cadmium, cobalt, chromium, copper, iron, lithium, magnesium, manganese, nickel, lead, tin, strontium and zinc (but not sodium and potassium) with higher fatty, resin, and naphthenic acids (for example, stearates, palmitates, oleates, linoleates, resonates, laurates, octanoates, ricinoleates, 12-hydroxystearates, naphthenates, tallates and the like). For further details concerning the term “metal salts” one can refer to Römpp's Chemical Lexicon, 10th edition, Georg Thieme Publishers, Stuttgart/New York, Volume 4, 1998, p. 2619, “Metal soaps,” and the literature cited there, where their disclosure in this regard is hereby incorporated in its full extent by reference.
However, it is known that slip agents that are based on metal soaps tend to have a strong plate-out effect. This means that even during processing, metal soap-based lubricants diffuse onto the surface of the plastics and cause a separating effect there between the plastic surface and a tool wall in contact with the plastic surface. This so-called plate-out effect is due to various degrees of incompatibility of the individual components of the mixture within the plastic mixture.
This is why plastics, before being processed, for example gluing, coating, printing and painting, generally require an activation of the surface, through which activation the wettability or the adhesiveness of the plastic surface for liquid glues, dyes, paints, inks and the like is first enabled. Such pretreatment measures in particular include corona pretreatment, flame pretreatment, chemical pretreatment, plasma pretreatment (for example, by means of a low-pressure plasma), fluoridation, etc.; as a consequence of the oxidation processes that take place the plastic surface becomes polarized. Another form of pretreatment, which is used, for example, in the automobile industry in painting polyamides, is the so-called priming of the substrate or plastic surface with a coupling agent in order to increase the affinity for the subsequent painting.
Plastics have been imprinted in this way for a long time now. For example, for many years polyamides have been printed, for example in screen printing, with light-curable, especially UV-curable inks, and UV-curable ink systems have also been long-established in paper and cardboard printing.
Nevertheless, the plate-out effect of the metal soaps used as lubricants in plastics that was noted above can be harmful for a subsequent printing, for example with a UV-curable ink. These metal soaps, for example metal stearates, also disrupt the printing process by the fact that they react with components of the UV ink system and/or form a barrier layer between the ink and plastic surface and thus diminish the functionality of said ink.
Light-curable inks in general contain one or more photoinitiators that are matched to a predetermined short wavelength of light. The light emitted during curing is chosen in a wavelength such that the photoinitiator is stimulated and the ink becomes at least partially polymerized; polymerization of the ink then leads to an increase of viscosity and to consolidation of the ink, where the degree of consolidation is dependent on the time of exposure and on the emitted radiation power. Current UV-curable inks are based in some cases on a UV light quantum-induced radical polymerization of acrylates (for example methyl methacrylate); acrylates have polar character because of the carboxyl groups that are present and thus have a certain surface tension, which is given in the literature as about 16 mN/m for the polar part. A certain affinity for a polar plastic, for example polar polyamide (surface tension of polar part about 18.1 mN/m) is said to be given by this. However, because of the so-called plate-out effect of the metal soaps that are used as lubricant (for example metal stearates) the surface tension of the plastic surface is reduced and printing is crucially degraded. This appears in a sharp difference of the quality between the “normal” state and the conditioned state. Compared to the “standard,” which already behaves qualitatively poorly, conditioned polyamides show a still much poorer contrast and still much poorer abrasion resistance, and these differences of quality are undesirable.
The invention therefore is based on the task of making available a method for improving the wettability and/or the adhesiveness of plastic surfaces (for example polyamide surfaces), in particular for purposes of easier capacity to be printed, painted or glued, which at least largely avoids the disadvantages of the prior art that were listed above or at least diminishes them.